Several industries use Radio Frequency Identification (RFID) systems for applications such as inventory control, asset verification, and vehicle movement tracking. RFID systems typically consist of two components, readers and tags. A reader is typically a wall-mounted or counter mounted device which broadcasts an interrogation signal to a tag and which can interpret the information from the tag for some use. Tags are typically small, plastic encased devices which are placed on or in an article carried by an individual and are used by several industries for applications such as inventory control, asset verification, and vehicle movement tracking. The tags typically contain information regarding the type of article or a unique identification number that is meaningful to the system. The information is obtained from the tag by interrogating the tag utilizing electromagnetic waves, such as magnetic, magneto mechanical, radio frequency (RF), microwave, and the like.
Most RFID systems use interrogation and response frequencies near or above 1 MHz, with the most commonly used frequencies being 126 KHz, 915 KHz, 13.56 MHz, and 2.45 GHz. At these frequencies, tags are susceptible to shielding, especially shielding by the human body such as a hand, which can be disruptive to their functioning.
Lower tag frequencies are moderately resistant to shielding by the human body and other objects, but have only limited radio frequency functionality due to bandwidth and distance limitations. In addition, the lower frequency tags are rather bulky. Bandwidth tends to increase and tags tend to be smaller as the operating frequency increases. In addition, interrogation distance is greatest at approximately 915 MHz and starts to go down again after 915 MHz. Unfortunately, these higher frequencies are more susceptible to shielding, especially by the human body.
There are two basic categories of tags: passive and active. Passive tags operate and respond to interrogation signals by absorbing the interrogation signal energy, and using this energy to wake up a semiconductor identification chip within the tag. The energy is also used to send back an electromagnetic signal that is often as little as 1/1000 the strength of the original signal. Active tags have a battery that is part of the tag that allows the return signal power to be amplified, thereby transmitting across greater distances.
RFID tags are typically interrogated by readers which have transmit and receive electronics that operate in the 0.1 to 0.5 milliwatt region. Tag interrogation typically occurs at distances ranging from 1 inch to 12 feet for passive tags. Active tags have much greater transmit distances, and can frequently transmit from 3 feet to 1000 feet or more and usually are much larger in size.
Radio frequency identification technology has been developed by a number of companies, including Motorola/Indala (see U.S. Pat. Nos. 5,378,880 and 5,565,846), Texas Instruments (see U.S. Pat. Nos. 5,347,280 and 5,541,604), Mikron/Phillips Semiconductors, Single Chips Systems (see U.S. Pat. Nos. 4,442,507; 4,796,074; 5,095,362; 5,296,722; and 5,407,851), CSIR (see European document numbers 0 494 114 A2; 0 585 132 A1; 0 598 624 A1; and 0 615 285 A2), IBM (see U.S. Pat. Nos. 5,528,222; 5,550,547; 5,521,601; and 5,682,143), and Sensormatic Electronics (see U.S. Pat. No. 5,625,341). The teachings of the above-referenced patents are included herein in their entirety.
Current reader and transmitter units are typically bulky and are not well suited for incorporation into a small low power device such as a wrist watch. Also, the typical 0.1 mw to 0.5 mw reader power requirements are not well suited for incorporation into a wrist watch, portable digital assistant (PDA), or other portable or wearable device.
In addition to practical problems faced by present RFID systems, including weak transmission signal power, greater size and weight associated with some tags and transducers, and signal shielding, other, more abstract problems also exist. One such problem is the fact that each country can control transmissions on various segments of the frequency spectrum within that country. Although there is some uniformity from country-to-country, it is not uncommon for different countries to have prohibited transmissions on frequencies typically used by present RFID systems. Thus, it is desirable to find a transceiver that will function at a frequency that will allow the transceiver, and the associated RFID system, to be used in a variety of countries.